Focus on nutritional elements: bicarbonate

28 february 2019

Bicarbonate is not a nutritional element, but it can occur in potting soil and have adverse effects on the culture. So it's still good to know what happens. Therefore, as a final entry in ‘Focus on nutritional elements’ the element: bicarbonate (HCO₃^-).

Bicarbonate is not a nutritional element and it is also not absorbed by plants. However, it can occur in potting soil. The occurrence of bicarbonate depends on the pH value in the soil moisture.

Although bicarbonate is not added to potting soil mixtures, it is mentioned on the nutritional analysis of potting soil. Bicarbonate in a potting soil has a number of origins: liming, carbonates in certain raw materials, the irrigation water used in the culture and it develops in the root environment from CO₂ (carbon dioxide) when the pH increases.

• Liming:

After liming of fresh potting soil, bicarbonate - at pH values higher than 5.5 - becomes visible in the soil analysis (1:1½ extraction with water). If there is still undissolved lime in the potting soil, then the bicarbonate content may rise slowly. However, the pH buffer of potting soil can diminish this process. Due to this buffer, the increase in the bicarbonate content and with that the pH value, can occur slightly or not at all. A potting soil with ‘firm liming’ in which a pH higher than 6.0 is desired, may show a higher bicarbonate content due to the higher pH achieved. In principle, bicarbonate may already develop starting at a pH of 4.0. However, this is so little that this will not be visible in an analysis.

• Carbonates in certain raw materials:

Raw materials such as clay and compost can contain carbonates, which ultimately contribute to a higher bicarbonate level. In a freshly produced potting soil, the bicarbonate content comes from partially decomposed carbonates.

• During the culture:

In a culture situation, the liming and the carbonates in raw materials no longer play a role after a while. This is because the carbonates neutralize the pH and are therefore ‘consumed’. During the culture, water origin and fertilization are important causes for an increasing bicarbonate content.
Spring water or surface water may contain high concentrations of bicarbonate. If water contains too much bicarbonate, growers generally acidify it.
Bicarbonate may also develop due to pH increasing fertilization. Plants then cause the pH to rise by absorbing more anions than cations. The increasing pH binds CO₂ - present in the root environment bacause of decomposition and root respiration - to OH^- (the negative hydroxide ion) and thus creates bicarbonate (OH^- + CO₂ = HCO₃^-). The best way to prevent this, is to adjust the fertilization to a more acidic composition. In general, the ratio between ammonium and nitrate in the fertilization is then changed to a higher ammonium content.

Due to bicarbonate or an increasing content of it, the pH in potting soil may also increase further. For plants, especially that rising pH is a problem. Too high a pH of potting soil means that the plants have a reduced or limited uptake of a number of nutritional elements. This can have various effects on the culture. The effects that a deficiency of nutritional elements can have in a culture is explained in all previously published entries of 'Focus on nutritional elements': www.rhp.nl/en/nutritional-elements

Bicarbonate is not added to potting soil mixtures. However, it is important to take into account the presence of it in potting soil, , especially since (an increasing content of) bicarbonate may have an effect on the pH. For the RHP quality mark, quality requirements are set for, among other things, the pH of potting soils. The potting soil producer and grower determine together the desired pH value, fitting the culture. The pH of RHP certified products is checked before delivery to the grower.